The strive toward compactness along with new power consumption rules requires integrated devices to perform functions in an increasingly intelligent manner. FIG. 1 depicts a flyback DC-DC converter in which the majority of the power and control components are integrated in an unique package, monolithic or hybrid.
Flyback converters may be classified as:
a) SOPS (Self Oscillating Power Supply) converters in which the demagnetization of the transformer on an auxiliary winding Aus is detected (verification of a complete transfer to the secondary circuit of the energy accumulated while driving the switch connected to the primary winding N1, during a subsequent off phase of the switch evidenced by the current across the primary winding becoming null so that the successive turning on of the power switch takes place with a null current in the primary winding), to control the turning on of the power transistor (POWER). In this way, a "discontinuous" functioning mode is realized which is different from a continuous functioning mode in which the power switch is turned on with current still flowing in the primary winding, and different from the discontinuous functioning mode that is imposed by altering the switching frequency as a function of the power absorbed by the load connected to the converter output (secondary circuit of the transformer). PA1 b) Fixed frequency converters which operate in a discontinuous mode under nominal operating conditions. However, under other conditions, such as for example, during start-up conditions and when recovering from short circuit events, they work in a continuous manner, unless the monitoring of the demagnetization is effected, typically on an auxiliary winding Aus, for disabling the functioning of the oscillator that establishes the fixed switching frequency.
One or the other configuration may be preferred depending on the application. In both cases there are control circuits CONTROL that carry out substantially the same functions. The integrated CONTROL circuit has a COMP pin, through which the information on the output voltage may be obtained by employing a photocoupler and to which an external capacitor, CCOMP, of a few hundreds nF is commonly coupled for compensating the output voltage control loop.
Moreover, such configurations include circuits that control the switching of the power transistor by employing a network operating on a pulse by pulse basis that limits the current in the power transistor, circuits that generate reference voltages REFERENCE, and histeresis circuits UNDERVOLTAGE. The UNDERVOLTAGE circuits define the start-up and recovery transients of the converter by intervening when the supply voltage VDD of the control circuitry, which typically is derived from the auxiliary winding Aus of the flyback transformer at steady state, is blown or accidentally drops below a certain threshold VDD.sub.off. In these events, the undervoltage circuit switches off the whole device and maintains it in such a disabled state until the voltage reaches or exceeds a second threshold VDD.sub.on higher than the former threshold, VDD.sub.off.
At power on, a charging current of the supply capacitor C2 (of the order of few tens of .mu.F) may be provided for a line connected in some way to the Valim node, for example by a resistor of adequate power dissipating characteristic and value. In certain cases such a charging line (resistor) may be integrated and for these reasons it is not shown in FIG. 1.
In the control circuits of SOPS converters there is also a pin DEM, for synchronizing the turning on of the power transistor (POWER) under demagnetization conditions of the transformer. In contrast, in fixed frequency converters, the turn-on synchronization of the power transistor takes place by an oscillator that produces a dedicated clock signal and for this purpose, a pin, Osc, is often reserved for setting the switching frequency by an external capacitor or R-C group. Commonly, these type of control circuits do not include the presence of a DEM pin, unless the use of a network for verifying the demagnetization of the transformer and enabling the turn-on when this condition is satisfied, is contemplated. Verification of this condition is often implemented for preventing a continuous mode of operation, which would require an oversizing of some power components of the converter. Commonly, CLAMPER or SNUBBER circuits are used to limit the maximum voltage value on the power transistor and/or to avoid overlaps of the current and voltage waveforms during switching.
Apart from the above mentioned undervoltage block, known converters do not have other protections against short circuit condition of the converter's output. In practice, if the output voltage becomes null because of a short circuit on the OUT terminals, in the secondary winding (and therefore on the diode D1) there will be, during the off phase of the power transistor, a current whose maximum value is given by: EQU Isec.sub.cc =(N1:N2) Ip.sub.max
The voltage mirrored on the auxiliary winding AUS, coincides with the voltage on the secondary (which during a short circuit, will be equal to the voltage drop Vf on the diode D1), multiplied by their turn ratio (N3:N2), that is: EQU V.sub.AUS(cc) =(N3:N2) V.sub.sec(cc) =(N3:N2) Vf (D1)
This voltage is commonly less than the lower threshold VDD.sub.off of undervoltage, and in these conditions, the diode D2 during the off phase is nonconductive and therefore, during the successive switching cycles the current is supplied by the capacitor C2 which being no longer charged, discharges itself until dropping to threshold VDD.sub.off of undervoltage.
If the short-circuit condition of the output persists, the VDD voltage will oscillate between the VDD.sub.on and VDD.sub.off determining a functioning as illustrated in FIG. 2, with an average duty-cycle established by the currents absorbed by the integrated control circuit in these two functioning modes. However, the lower threshold of the undervoltage circuit may never be reached under short-circuit conditions, unless certain circuit arrangements are implemented in the converter scheme. The presence of parasitic inductances of the transformer cause some damped oscillations during the turn-off phase of the power transistor, whose maximum peak may be high enough to maintain a charge state of the C2 capacitance above the threshold VDD.sub.off of undervoltage, which keeps the device permanently turned on and subjects the power transistor to continuous and very demanding switching cycles. In certain situations, this may lead to the destruction of the device.
To prevent this, it is a common practice to add protecting components, external to the integrated circuit.
Known approaches have drawbacks. Firstly, the condition of short circuit is indirectly recognized by a discharging of the supply capacitor, with the consequent turning off of the device upon reaching the lower threshold of undervoltage. The power dissipated in such conditions is clearly tied to the charge and discharge transients of the supply capacitor. Secondly, it is necessary to use external components for implementing such a turn-off function. Indeed, the realization of an effective protection by a wholly integrated circuit appears very difficult because in these integrated circuits it is not possible to effect a correct control of the output voltage, and the short circuit condition may be indistinguishable from that of a normal start-up condition with a completely discharged output filter capacitor.